Primary Article:

Emerton, R., Nicolas, J., Lombardi, A. et al. Global heat stress intensification and its expanding footprint on the human population. Nat. Clim. Chang. (2026). https://doi.org/10.1038/s41558-026-02670-5

Introduction

As time progresses, climate warming is resulting in more frequent and intense heat extremes. These rising temperatures further increase the risk of heat-related illnesses which in turn can exacerbate already preexisting conditions such as respiratory and cardiovascular diseases.

Heat stress, such as heat stroke and heat exhaustion, is the leading cause of weather-related mortality, but changes in heat stress remain poorly quantified at a global scale. Better understanding the changes in heat stress can result in improving how we protect public health by minimizing physical and cognitive health disruption.

Fluctuations of Heat Stress Since the ’70s

A thermal stress index is a way to measure how environmental conditions combine to affect the human body’s ability to regulate its temperature. In the study entitled, “Global Heat Stress Intensification and its Expanding Footprint on the Human Population”, researchers utilized a widely adopted thermal stress index known as the Universal Thermal Climate Index (UTCI) to analyze the historical changes in heat stress overtime along with the global population’s exposure to heat stress. The UTCI accounts for temperature, humidity, wind, and radiation along with modeling how the human body responds to the environment. This modeling is expressed as a ‘feels-like’ temperature.

In this study, research was completed by taking a holistic approach looking at multiple interacting systems known as multidimensional intensification. Researchers also reviewed both daytime and nighttime temperatures. An overly hot night is referred to as a tropical night where the minimum temperature does not drop below 20 degrees Celsius. These tropical nights are dangerous because they deprive the body of the natural cooling-off period to recover from daytime heat stress.

A main finding from this study was major changes in frequency, severity, and duration of heat stress. It has been concluded that heat stress days and tropical nights have risen exponentially since the 1970s. Along with the population’s exposure to heat stress and durations increasing significantly over the last decade.

The figure below looks at the change in frequency of compound heat stress days and tropical nights from the 1970s and the last decade. There are eight different graphs looking at consecutive heat stress days and tropical nights in different parts of the globe. Values in red indicate an increased frequency over the last decade while blue values indicate decreased frequency.

From the previous figure it is concluded that more frequent heat stress days and tropical nights are pushing an increase in compound heat events which have globally increased and prolonged especially in Europe, Africa, and North America.

The figure below shows the percentage of the global population exposed to heat stress from the 1970s and the last decade. Gray bars indicate the population percentage exposed to each threshold and duration of heat stress which parameters are shown on the x and y axis. Light blue bars are the increase in thresholds due to population growth over the last decade. Teal bars indicate the increase from heat stress changes.

From the above figure, it can be concluded that exposure to at least one extreme heat stress day has grown, also considering the growing population over the last decade. Increase in exposure for most categories has increased as well.

Further Peril That Follows Closely Behind Climate Change

 This study provided insight into changes in the severity, duration, and frequency of heat stress. Results have proven an increasing climate-driven intensification of heat stress with implications for affecting global health of the human population overtime.

Not only do these rising temperatures have an impact on human health, but the environment we rely on as well. Resulting in more frequent incidences of extreme weather events such as flooding in some regions and extreme drought in others. Heat stress indirectly diminishes water availability and the productivity of crops and livestock.

Future research could delve into the specifics of how climate change and the increased exposure to heat stress effects human health and open up ideas on how to combat increasing heat stress.

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